Add cargo contract generate-interface --target solidity

[cmichi]

Motivation

For proper cross-contract communication between ink! and Solang-compiled Solidity contracts we need the ability to generate a Solidity interface for a contract.

ToDo

As part of this issue a new sub-command cargo contract generate-interface should be added. This command should have two flags:

• --metadata-path: the contract to which we want to generate an accessor interface.
• --target: denoting which language the interface would be generated in. For this issue we only add solidity as a target, outputting Solidity code for accessing the contract from the metadata.

[xermicus]

Related: hyperledger/solang#976